Implementation of string arguments of the form \*[foo arg1 arg2 ...]

Implementation of string arguments of the form \*[foo arg1 arg2 ...]

* src/roff/troff/input.cc (have_string_arg): New global variable.
(read_mode): New enumeration.
(read_escape_name): Use it.  Update all calls.
(read_long_escape_name): Use it.  Update all calls.
Set have_string_arg if appropriate.
(get_char_for_escape_name): Add parameter for handling space
character.
(interpolate_string_with_args, decode_string_args): New functions.
(get_copy, token::next): Call it if necessary.
(interpolate_string): Fix error message.

* NEWS, doc/groff.texinfo, man/groff.man, man/groff_diff.man:
Document it.